Presently, alarm and on-off control functions are accomplished through the use of cam-operated snap action switches, or magnetically via mercury switches. Switching functions also have been accomplished optically with an incandescent light source with individual photodetector set points. In one such gauge, relays are activated when the photodetectors are shielded from the light source by an opaque ribbon which is mechanically pulled by the pointer mechanism.
The operation of all of the above devices is such as to tend to distort the accuracy of the gauge or meter due to a relatively high degree of mechanical loading of the pointer drive mechanism. Another disadvantage is that only a limited number of points may be used on a given gauge.